This invention relates to a new process of fracturing a carbonate reservoir in a subterranean formation to stimulate the production of hydrocarbon fluids from the formation. During the process, the composition and reactivity of the fracture stimulation fluid that is injected into the formation surrounding a wellbore is varied from a lower reactivity fluid to a higher reactivity fluid. The new process is designed to effectively stimulate the fracture starting from the tip of the fracture and progressing back to the wellbore.
Fracture stimulation, commonly referred to as fracture acidizing, or acid fracturing, when acid is the stimulation fluid, is a stimulation technique commonly used to increase the productivity of hydrocarbon fluids from subterranean formations. Fracture acidizing is used in carbonate reservoirs. The technique typically involves the injection of acid, usually aqueous hydrochloric acid (HCl), through a wellbore and into the formation at pressures sufficient to fracture the formation or open existing fractures. The acid etches the fracture faces, resulting in the formation of conductive flow paths. Frequently, the treatments are not effective. The depth of stimulation is typically limited by rapid consumption of acid near the wellbore and loss of acid through the fracture faces (commonly referred to as fluid leakoff or fluid loss). Fluid leakoff is a dynamic process that is influenced significantly by the formation of wormholes that form in the porous walls of the fracture. Wormholes are highly conductive flow channels that form approximately normal to the fracture. These wormholes divert fluid from the fracture, consume large amounts of reactant from the fracture stimulation fluid, and provide no benefit to the conductivity of the fracture. By xe2x80x9cconductivity of the fracturexe2x80x9d is meant the capability of formation fluids to migrate or flow through the conductive etched flow channels that are formed by the reaction of the fluid with components of the formation along the faces of the fracture. The formation fluids, of course, migrate or flow through such conductive etched flow channels to the wellbore where they are produced to the surface and recovered. The creation of such conductive etched flow channels in the formation is easily evidenced by enhanced production of formation fluids from the well, and such channels can also be visually observed in the laboratory using conventional acid conductivity tests on core samples.
Fracture stimulation fluid systems, such as emulsified HCl, have been devised which tend to provide deeper penetration of live acid. The effectiveness, defined based on the depth of live acid penetration, of such systems in fracture acidizing treatments is enhanced because the rate of dissolution and rate of wormhole propagation are decreased relative to straight HCl. However, near wellbore conductivity is typically low due to insufficient dissolution or etching of the fracture faces that, in turn, is caused by an initial cool-down effect and fracture geometry in the near wellbore vicinity. Thus, a method of increasing both the length and conductivity of the conductive etched flow channels is required to improve the effectiveness of fracture stimulation treatments.
A novel process of fracture stimulation has now been discovered to stimulate the production of hydrocarbon fluids from carbonate reservoirs in subterranean formations penetrated by a wellbore. The new process comprises injecting a fracture stimulation fluid into and through a wellbore and into the carbonate reservoir under pumping conditions that are selected and controlled to maintain an optimum fracture stimulation efficiency number, Ff, of about 0.1 to about 0.3 during the fracturing process. The fracture stimulation efficiency number in the present invention is selected and controlled such that the fracture is effectively stimulated starting from the tip of the fracture and progressing back along the fracture to the wellbore. The fracture stimulation fluid compositions and treatment conditions used to maintain the optimum fracture efficiency number can be conveniently regulated by varying the reactivity of the fracture stimulation fluid from a composition of low reactivity to one of higher reactivity during the process. The flow rate and/or viscosity of the fracture fluid can also be varied to control the rate of mass transfer of the reactants and products in accordance with an optimum fracture stimulation efficiency number, based on formation and fluid parameters. The new fracturing process can provide deep penetration of live reactant along the fracture, reduce the rate of wormhole formation to control fluid loss, and efficiently create highly conductive etch patterns on the fracture faces.